HERA

Allen Caldwell Max Planck Institute for Physics

Physics case high energy e/p and e/A collider

Alegro Workshop CERN

March 26, 2019

VHEePfundamental state of QCD matter?

relation to gravity? color confinement?

!2

To the outside world, the proton is made of three quarks, two ‘u’ and a ‘d’.

Other quarks which have been discovered: s,c,b,t

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Feynman’s Parton Model

Imagine a very fast moving nucleon. Feynman asked us to think of it as a collection of point-like constituents- the partons- which are behaving incoherently.

e

p

γ

All partons moving parallel to the nucleon. Assume massless and no significant transverse momentum. Parton carries fractional momentum

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xi = 1i∑

Interactions between partons in this frame time-dilated, so wavefunction not changing during the time of the interaction.

4

s=(k+P)2 CM energy squared Q2=-(k-k’)2 virtuality W2=(q+P)2 γ*P CM energy squared

Transverse distance scale probed:

McAllister, Hofstadter Ee=188 MeV bmin=0.4 fm Bloom et al. 10 GeV 0.05 fm

CERN, FNAL fixed target 500 GeV 0.007 fm HERA 54 TeV 0.0007 fm

LHeC/PEPIC 900 TeV 0.0002 fm

VHEeP 45000 TeV 0.00003 fm

Kinematics

*

x ≈Q2

W2

b ≈ℏcQ

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e

McAllister and Hofstadter 1956

188 MeV and 236 MeV electron beam from linear accelerator at Stanford

Conclusion: radius of proton 0.7-0.8 fm

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Bloom et al. (SLAC-MIT group) in 1969 performed an experiment with high-energy electron beams (7-18 GeV).

W=0.8 2.0 3.6 GeV

At small W, see proton and resonances.

The data did not fall with Q2 as expected for elastic scattering on an extended nucleus. Rather, data looks like elastic scattering on a point charge (Mott).

Scaling observed Predicted by Bjorken (1967). Implies electron scattering on point-like objects.

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x q (x) + q(x)[ ]q∑∫ dx < 1

from the Review of Particle Properties, Phys. Lett., 170B (1986) 79.

After discovery of quarks, a period of inelastic lepton-nucleon studies ensued:

Beams: electron,muon, neutrino

Targets: p, D, heavy targets

QCD was established as the correct description of the strong interactions.

Main results: There is something else in the proton than quarks – gluons !

A. Caldwell, MPI f. Physik11 March, 2016 !8

HERA physics (1992 – 2007 + …)

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.

.~0.3 fm

~1900 ~1970todayHERA Highlights

Proton full of partons when viewed with high resolution probe!

F2 structure function

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≥10% of events, protons survive collision! Implies scattering on color neutral cluster: at least two gluons. Sometimes called ‘Pomeron’.

η≈y

xBj

!d

iff(

bin

")/!

tot

"=0.40H1 data 99-00 (prelim.)

H1 C

ollab

ora

tio

n

0

0.01

0.02

0.03 Q2=15 GeV

2Q

2=20 GeV

2

0

0.01

0.02

0.03 Q2=25 GeV

2Q

2=35 GeV

2

0

0.01

0.02

0.03

10-4

10-3

10-2

Q2=45 GeV

2

10-4

10-3

10-2

Q2=60 GeV

2

Ratio of diffractive/total independent of x at small x. Small-x physics is universal.

.

. proton survives collision

11

Not sensitive to details of proton structure

Start to see quarks – the valence quarks

See that valence quarks have complicated structure

What’s next ?

Higher energy allows finer probe

New Physics in QCD

color confined ? mass of the proton = 1 GeV ? spin of the proton = 1/2 ?

asymptotic freedom ?

The interesting features of QCD are not apparent from the QCD Lagrangian - emergent features

Fundamental to understand how they emerge: •most of the visible mass of the universe

comes from QCD binding energy •QCD is at the heart of everything (photons,

electrons,…). What does matter look like in the high energy limit ?

• confinement of color at 1fm distance scale and spin structure - result of complicated multi particle nonlinear interactions

New ep/A colliders tasked with bringing insight into these important questions

Future Collider Options - EIC

Will be the first eA collider First time both e,p polarized high luminosity. Realization: late 2020s

Future Collider Options - EIC

Study spin structure

small-x in nuclei

12

=12

ΔΣ(Q2f ) + ΔG(Q2

f ) + LQ+G(Q2f )

How does spin 1/2 emerge from quark, antiquark, gluon and orbital angular momentum? Key: both beams polarized, lots of events, small-x

5D structure

Key: lots of events, full acceptance detector

Probe scatters off partons from different nuclei:”oomph factor”. Key: maximum energy

borrowed from J. Qiu DIS 2017, Kobe, Japan

Future Collider Options - LHeC

60 GeV electrons x LHC protons/ions high luminosity 1034 cm-2 s-1

Simultaneous running with ATLAS / CMS in HL-LHC period ?

Broad physics program:

Small x; novel QCD / unitarityMedium x: precision Higgs and EW

High x:new particle mass frontier

Per-mille experimental precision on αS

LHeC

1) CDR 2012

2)  TDR for PERLE

3)  Further development of FCC eh

Future Collider Options - LHeC

x7−10 6−10 5−10 4−10 3−10 2−10 1−10 1

xf(x,Q)

0

0.5

1

1.5

2

2.5(x,Q)cx(x,Q)sx(x,Q)ux(x,Q)dx

xg(x,Q)xd(x,Q)xu(x,Q)xs(x,Q)xc(x,Q)Q = 1.41e+00 GeV

LHeC pdf

0

1

2

3

4

5

6

7

8

WW ZZ gg γγ cc tt bb μμ ττ

CMS-S1

CMS-S2

LHeC

ep+pp

δκ/κ[%] preliminary

Precision proton structure functions important in recognizing new physics

small-x physics: vastly extended high lumi: exclusive observables !

High lumi: sensitivity to Higgs couplings

eA will also be possible

Future Collider Options - PEPIC

Create ~50 GeV beam within 50−100 m of plasma driven by SPS protons luminosity expected to be << 1030 cm-2 s-1. Focus on QCD Studied within Physics Beyond Colliders workshop @ CERN. Relatively inexpensive first application of PWA ?

PEPIC

Physics: focus on QCD processes large cross sections, growing with energy -> luminosity requirement modest

saturation?See Froissart behavior ?

VHEeP (Very High Energy electron-Proton collider)

One proton beam used for electron acceleration to then collide with other proton beam

Luminosity ~ 1028 − 1029 cm-2 s-1 gives ~ 1 pb−1 per year. Studies on achievable luminosity ongoing.

Electron energy from wakefield acceleration by LHC bunch

Choose Ee = 3 TeV as a baseline for a new collider with EP = 7 TeV yields √s = 9 TeV. Can vary. - Centre-of-mass energy ~30 higher than HERA. - Reach in (high) Q2 and (low) Bjorken x extended by ~1000 compared to HERA. - Opens new physics perspectives

Mini-workshop on QCD and GravityDecember 12,13

Max Planck Institute for Physics

Wednesday, December 12

14:15-15:15 Raju Venugopalan ‘A many-body theory of QCD in the Regge limit’

15:30-16:00 Eran Palti ‘News on Swampland’16:00-16:30 Stephan Stieberger ‘QCD meets Gravity’16:30-17:00 Johanna Erdmenger ‘AdS/CFT and very small x’17:00-17:30 Angnis Schmidt-May ‘News from bimetric gravity’17:30-18:30 Discussion time

19:00- Dinner somewhere

Thursday, December 13

9:00-10:00 Gia Dvali et al ‘Proof of the Axion?’10:00-10:30 Discussion time10:30-11:00 Henri Kowalski ‘BFKL analysis of HERA data’11:00-11:30 Agustin Sabio Vera ‘The Regge limit in QCD, SUSY and gravity’

12:00-14:00 lunch and discussion

Prospects for a very high energy eP and eA collider

June1,22017MaxPlanckInstituteforPhysics

2 workshops to discuss novel physics with very high energy eP/eA collider.

Second - focus on relation between QCD and gravity.

2018

Small-x physics and color confinement. One of the key unsolved questions in physics.

How? R. Venugopalan, Mini-workshop on QCD and Gravity December 12,13 Max Planck Institute for Physics

New: perturbative approach to infrared physics! Relevant equations very similar to fundamental statistical mechanics equations. Strong overlap with quantum description of black holes.

From the physics we know - these are the strongest fields that can be achieved in nature

VHEeP

with VHEeP and eA, we will be in a region where the saturation scale is well into the perturbative region. Allows detailed probing of this new physics: high density & weak coupling !

At high energy, see short-lived fluctuations due to time dilation

Courtesy: R. Venugopalan

Markovian process leads to power law growth of gluon distribution at small x

Violates Froissart bound asymptotically

Afascina)ngequilibriumofspli4ngandrecombina)onshouldeventuallyresult.Itisaconsiderabletheore)calchallengetocalculatethisequilibriumindetail...

F.Wilczek,Nature(1999)

QS2~A1/3since

”wee”gluonscouplecoherentlyforx<<A-1/3

“oomph factor” from Nuclei

QCD and Gravity: more than math ?

Consider: the visible mass is largely due to baryons. The mass of baryons is largely due to QCD (not the Higgs mechanism). Gravity couples to mass ….

S. Stieberger, Mini-workshop on QCD and Gravity December 12,13 Max Planck Institute for Physics

April4,2017

Huge cross section – no statistical uncertainty even at 1028 cm-2s-1

What will we measure and why will it help?

Total photoproduction cross section – energy dependence ? See approach to Froissart bound ? Impact on cosmic ray physics

Virtual photon cross section: unphysical extrapolation of cross sections -> observation of saturation of parton densities ?

With the three orders of magnitude extension in the range at small-x, expect to see signs of the fundamental saturated regime.

10-1

1

10

102

103

104

105

1 10 102

103

104

Photon-proton centre-of-mass energy, W (GeV)

σ (

γp →

Vp

) (

nb

)

φ

Υ(1S)

σtot

ρ

ψ(2S)

J/ψ

ω

W0.16

W0.22

W0.22

W0.7

W0.7

W1.0

VHEeP reach

H1/ZEUS

fixed target

ALICE/LHCb

VHEeP, √s

Exclusive processes: Sensitive to square of gluon density Early signs of new saturated regime

Good opportunity to see the fundamental high-energy saturated state!

Crossing unphysical: new physics needed

eA possibility will make this physics even more dramatic “oomph”-factor again

q

e±

q

LQ

P

LQ

P

γ, Z,W

q q

P

e±

q

e±

q

e±, νe

e±, νe

e±, νe

(b)(a)

(c)

At high energy - leptoquarks appear ?

tailor-made for eP Collider

MLQ (TeV)

ν / λ

90 % upper limit ν90 % upper limit λ

10-2

10-1

1

10

1 2 3 4 5 6 7 8 9

Simulation study: 100 pb-1

Sensitivity well beyond expected reach of LHC

High energy electron-proton and electron-ion collider

new conceptual breakthroughs, (probably) not new particles

• where mass comes from (saturated high energy state of QCD matter). • approach color confinement via large saturation scale • connections to gravity may become apparent • and lot’s of ‘bread-and-butter’ physics